After being announced in December, mobile gaming backend service OpenKit plans to bring in the first 50 developers into its private beta program tomorrow.

Co-founder Peter Relan told me that when the team first publicized the project last month, “not a single line of code” had been written. Instead, it was sussing out developer interest, with plans to focus on the project if more than 100 developers signed up. In fact, 160 developers signed up on day one, and that number is now up to 500.

OpenKit is, in large part, a response to the shutdown of OpenFeint, a social platform for mobile games that Relan co-founded, and which had been acquired Gree. Even beyond OpenFeint, Relan said that developer lock-in seemed to be a big theme in 2012.

So OpenKit is an attempt to create a platform that’s both “good business and developer friendly.” Specifically, the code is being developed as an open source project on GitHub, and game developers will also be able to export their user data from OpenKit at any time — so if they want, they can take the code and their data and create their own backend service.

Relan described the version launching tomorrow as a minimum viable product with just a few core features in place — cloud storage (so a player can save their game on one device, then load it up on another), player leaderboards, and player authentication using Facebook and Twitter. The plan is to go into public beta in February, and eventually to make money through a freemium business model, where the basic tools are free but developers have to start paying when they reach a certain amount of usage.

The landscape has changed since the launch of OpenFeint, Relan said. Previously, the most-demanded feature was player leaderboards, but this time it’s cross-platform cloud storage. He suggested that’s because it’s increasingly important to take Android seriously — where most of the initial OpenFeint developers were building exclusively for iOS, 80 percent of the ones signed up for OpenKit are cross-platform.